Gem Stones of the United States

Gem Stones of the United States GEOLOGICAL SURVEY BULLETIN 1042-G A CONTRIBUTION TO ECONOMIC GEOLOGY GEM STONES OF THE UNITED STATES By Dorothy M. Schlegel ABSTRACT Many semiprecious, but few precious, gem stones have been found in the United States. Beauty, durability, and rarity are the most important qualities of a precious gem. Gem stones are distinguished by their, physical properties: color, crystal form, cleavage, parting, hardness, specific gravity, luster, index of refraction, transparency, and dispersion. Gems are named for their color, type locality, outstanding physical property, or persons. The most popular gem cuts are the cabochon, rose, brilliant, step, and mixed. The carat, one-fifth of a gram or 200 milligrams, is the unit of weight measure ment. The color of four popular gems may be changed by heat treatment or dyeing. Only the ruby, sapphire, spinel, emerald, rutile, and quartz of gem quality have been synthesized. The best quality of assembled stones are the doublet and triplet. Most gem stones are found in alluvial gravels and igneous rocks, especially granite and pegmatite deposits. INTRODUCTION Gem stones generally are divided into two categories: precious and semiprecious. A precious gem stone has beauty, durability, and rarity, whereas a semiprecious gem stone has only one or two of these qualities. The diamond, emerald, ruby, and sapphire are considered precious gems. Some opal is precious, but most varieties are semiprecious. The beauty of a gem stone is determined by personal taste. In ancient times man preferred brightly colored, translucent or opaque stones. Today he prefers .evenly tinted, transparent stones. The desired hues are blue, rose, green, and true canary yellow in the diamond; pigeon-blood red in the ruby; cornflower blue in the sapphire; and grass green in the emerald. Most diamonds, however, are colorless. The durability of a gem stone depends upon its hardness and lack of ready cleavage. A gem must be sufficiently hard to resist abrasion by objects normally found in everyday life and by dust. It should also resist the chemicals withwhichitcom.es in contact. Cleavability is the tendency of certain gems to split in one direc tion more readily than in another. Rarity is one of the most important factors in establishing the price of a gem stone. Such gems as the diamond and ruby are rare, in addition to being beautiful and durable, and therefore are very expensive. Although the deep redpyrope garnet closely re sembles the ruby in color, there is no comparison in expense and popularity. 203 CONTRIBUTIONS TO ECONOMIC GEOLOGY .The purpose of this report is to give the amateur gemmologist some of the important information available on the gem stones of the United States. Although the finest precious gems occur in foreign countries, a wide variety of semiprecious stones, and a few precious gems, have been found in the United States. About 50 major gems and the geology of their occurrence are described in this report. William F. Foshag, U. S. National Museum, has kindly reviewed the manuscript and made many valuable sugges tions. Dana's "A textbook of mineralogy" is the source of the chemical formulas. PHYSICAL PROPERTIES Physical properties are important aids in identifying gems. They are: color, crystal form, cleavage, parting, hardness, spe cific gravity, luster, index of refraction, transparency, and dis persion. Color is the first property noticed in a gem stone. Minerals of constant chemical composition usually have definite, character istic colors. Many minerals, however, have varying chemical compositions and therefore a wide range of colors. Impurities, such as iron, chromium, and copper, also tint the minerals. Care should be taken in using color as a means of gem identification be cause it is not a constant property in most minerals. Most gem stones have certain crystal systems that form under favorable conditions of temperature and pressure. If the condi tions of formation are unfavorable, -an amorphous mass having no particular atomic arrangement results. The six crystal systems in which gem stones may form are: isometric, hexagonal, tetrag onal, orthorhombic, monoclinic, and triclinic. Cleavage is.the tendency of certain gem stones, when struck in the proper place, to split in one direction more readily than in another and yield plane surfaces. These plane or cleavage, sur faces are parallel to some of the crystal faces of some gem stones. For example, diamonds maybe cleaved in four directions parallel to the faces of an octahedron. Ready cleavage, however, can be a handicap; topaz and kunzite, for example, are difficult to cut into faceted gems because of this factor. Certain minerals form planes of structural weakness if exposed to Stress or pressure. These planes are known as parting. Twinned crystals and certain minerals that have been subjected to the critical pressure are usually affected in this manner. Hardness is the resistance a gem stone offers to abrasion or scratching/ The hardness of a gem stone may be determined by scratching it with another mineral or with a knife blade. This may mutilate tllQ StOne. A Series of 10 minerals, known as Moh's Scale of hardness, has been established to show relative of hardness. GEM STONES OF THE UNITED STATES 205 MOH'S SCALE OP HARDNESS 1. Talc 6. Orthoclase 2. Gypsum 7. Quartz 3. Calcite . 8. Topaz 4. Fluorite 9. Corundum 5. Apatite 10. Diamond The numbers preceding each mineral indicate only increasing degrees of hardness, not actual values. For example, the dia mond, rated as 10, is many times harder than corundum, rated as 9. Corundum is placed next to diamond only because it is the mineral nearest the diamond in hardness. Because much dust is composed of silica (quartz, 7 on Moh's scale) particles, a gem stone should be harder than quartz. Some minerals are harder in one direction than another. When testing a gem for hardness, the gem stone should be rubbed against the test stone, not vice versa. Cut stones can be badly damaged by permitting several of different hardness to be carried in con tact with one another. The specific gravity of a mineral is the ratio between its weight and the weight of an equal volume of water at 4° C. It is a con- stantproperty of all minerals of fixed compositior The amateur gemmologlst need not determine the specific gravity accurately, but he should learn to compare the relative weights of stones. Luster, the general appearance of a mineral in reflected light, is of two types: metallic and nonmetallic. Most gem stones are nonmetallic. Terms used to.describe nonmetallic lusters are: vitreous or glassy, resinous, pearly, greasy or waxy, silky, and adamantine or brilliant. Luster is influenced by the index of re fraction, transparency, and dispersion of a gem stone. When light rays pass from air into a denser medium, they are bent or refracted. The velocity of light is diminished in the denser medium. The ratio between the velocity of light in air and that in the gem stone is known as the index of refraction. If a gem is transparent, the maximum amount of light is refracted. In translucent or opaque stones only part of the light is refracted. Dispersion is the ability of a mineral to separate white light into colored rays. Because the degree of refraction for different colors of light differs with each mineral, certain minerals have a higher dispersion than others. The diamond owes its fiery bril liance to a high index of refraction and high dispersion. TECHNOLOGY GEM NAMES In ancient times gem names were based principally on color. The term "ruby" was applied to all red stones: spinel, ruby, garnet, and rose tourmaline; the term "sapphire," to lapis lazuli and other blue stones; the term "topaz," to all yellow stones; and 206 CONTRIBUTIONS TO ECONOMIC GEOLOGY the term "emerald, " to chrysocolla and all other green stones (Smith, 1949, p. 175). Corundum was referred to as "oriental" because the first specimens were brought to Europe from the Orient. Modern nomenclature for gems is based on the color, type locality, or an outstanding physical property of the gem. Several gems, such as uvarovite and kunsite, have been named for persons. GEM CUTS Most gem stones used in jewelry must be cut to reveal their full brilliance. In ancient times a gem stone was treasured for its surface coloration rather than its brilliance; therefore, gems were merely polished and left in their natural state. / The oldest, type of gem cut in use today is the cabochori or rounded ut introduced by the Romans (Smith^l949, p. 141). The cabochon Was used for colored stones, such as the emerald, ruby, sapphire, and garnet, but is now used only for-sta-rstones, cat's-eyes, opals, and generally opaque and imperfect stones. The art of faceting a gem stone originated in India in the 15th century (Smith, 1949, p. 142). The first faceted gems were dia monds that had been haphazardly cut to disguise any flaws or car bon spots. In the early 17th century the rose cut, a symmetrical form consisting of a flat base and a faceted top terminating in a low point, was invented and used for cutting large diamonds. This form was varied in the Dutch rose, Antwerp rose, and rose recouped. The brilliant cut, consisting of 56 facets plus the table andculet, was invented at the end of the 17th century. It revealed the fiery brilliance of a diamond for the first time and is still the standard cut for the diamond. Modifications of the brilliant form are the jubilee cut (88 facets), English brilliant (30 facets), the marquise, and the drop form. The other faceted forms are the step cut, used for emeralds and exceptionally good diamonds, and the mixed cut, a combina tion step cut and brilliant cut.

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